A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane 102 which moves the bones of the middle ear 103 that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external signal processor 111 in which various signal processing schemes can be implemented. The processed signal is then converted into a digital data format for transmission by external transmitter coil 107 into the implant 108. Besides receiving the processed audio information, the implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple electrode contacts 112 on its surface that provide selective stimulation of the cochlea 104.
After an electrode array has been implanted, the body can react by forming fibrous tissue around the array. This adversely affects the impedance and charge transfer from the electrode contacts, and thus should be avoided or minimized One way to do that is to form a layer of hydrogel material over the electrode contacts. Such hydrogel material is biocompatible and electrically conductive so as to allow for the intended charge transfer from the electrode contact to the adjacent tissue. But the hydrogel material also prevents the direct contact of the metal material of the electrode contacts (e.g., platinum) with the cochlear tissue and thereby avoids formation of the undesirable fibrous tissues over the electrode contacts. See, for example, U.S. Pat. Nos. 5,786,439, 7,519,435, 7,519,435, 8,190,271; which are incorporated herein by reference.
The hydrogel materials swells when it contacts the perilymph fluid within the cochlea, absorbing more than its own dry weight. As this swelling occurs, polymer branches in the hydrogel matrix grow much larger, forcing the hydrogel material away from the electrode surface it lies against. The chemical bond that normally is used to connect the hydrogel material to the electrode array often is not strong enough to resist these swelling induced forces. When that happens, the hydrogel material separates from the electrode array and can undesirably wander away from the implanted array. One solution to this is described in the priority application, U.S. Provisional Patent Application 61/874,388, filed Sep. 6, 2013, which is incorporated herein by reference in its entirety.
Cochlear implants system exhibit high overall reliability. One cause of the rare failures in such systems is the occurrence of open circuits within the electrode array. See Carlson et al., Prevalence and Timing of Individual Cochlear Implant Electrode Failures, Otol Neurotol. 2010 Aug. 31 (6):893-8; which is incorporated herein by reference in its entirety. The main cause of open circuits within cochlear electrode arrays is externally applied force to the metal wires within the silicone array carrier. These forces can occur momentarily with a high amplitude due to an accident, or they may occur chronically at lower amplitude due to micro-movements induced by muscular activity. These forces applied to the metal structure of the electrode wires results in material fatigue and ultimately a mechanical failure, i.e. wire breakage.
To divert/distribute externally applied forces, the electrode wires can be wave-shaped so that induced force moves the entire flexible electrode array and only a fraction of the external force energy directly affects the metal structure of the electrode wire. See U.S. Pat. No. 8,112,161, which is incorporated herein by reference in its entirety. It also is known to achieving a certain stretch-ability by means of defined microstructures such as by thin-film technique, ribbons or grapheme. See, e.g., Someya, Takao (Editor), Stretchable Electronics, Wiley, 484 pages, December 2012, ISBN: 978-3-527-32978-6; which is incorporated herein by reference in its entirety. Some cochlear implant systems also implement multiple redundant electrode contacts and/or electrode wires within the electrode array to allow for the deactivation of an affected electrode(s) without a loss of clinical benefit of the cochlear implant. But if the loss of the clinical benefit is too large, then a revision surgery can be performed, to replace the defective part.
Sometimes, cochlear implant electrodes can contain drugs or chemicals. To avoid ototoxic reactions (i.e. damage to the inner ear due to unintended exposure of drugs or chemicals), the electrode wires are encased in a silicone array carrier and the electrode contacts are made of platinum. See, e.g., Stöver et al., Biomaterials in Cochlear Implants, GMS Curr Top Otorhinolaryngol Head Neck Surg. 2009; 8: Doc10; which is incorporated herein by reference in its entirety.
An alternative approach is taught in U.S. Patent Publication 20130006339 and U.S. Patent Publication 20090171336, which disclose metal wires comprising liquid metal instead of solid metal. But, these publications do not address problems that can arise when such liquid metal wires break.